1. Field of the Invention
The present invention relates to a power amplifier that can analogically control the idle current of an amplifier element corresponding to a control voltage, and also can switch bias modes corresponding to the control voltage.
2. Background Art
Presently, as CDMA and other power amplifiers for mobile phones, GaAs-HBT (hetero-junction bipolar transistor) power amplifiers are widely used. FIG. 15 is a circuit diagram showing a conventional GaAs-HBT power amplifier. The circuit in the dotted-line frame is the GaAs chip, and other circuit elements are formed by chip parts and lines on a module substrate.
In FIG. 15, Tr1 and Tr2 denote GaAs-HBTs, which are a former amplifier element and a latter amplifier element, respectively. Bias1 is a former bias circuit that drives the former amplifier element, and Bias2 is a latter bias circuit that drives the latter amplifier element
Vc1 and Vc2 denote collector power source terminals for the former and latter amplifier elements, respectively; Vcb denotes a power source terminal for the bias circuits Bias1 and Bias2; and Vref denotes a terminal to apply control voltage to the bias circuits Bias1 and Bias2. IN denotes an RF signal input terminal; OUT denotes an RF signal output terminal; Rb1, Rb2, Rb12 and Rb22 denote resistors; C1 to C4, C21 to C23, Cd1, Cd2 and Cdb denote capacitors; and L1 and L2 denote inductors. L11 and L21 to L23 denote lines having specific electrical lengths and function as inductors.
FIG. 16 is a circuit diagram showing a conventional power amplifier. The bias circuit for the power amplifier is the above-described latter bias circuit Bias2. However, the former bias circuit Bias1 can also be used. In FIG. 16, Vrefb denotes a terminal to which reference voltage is applied from the outside; Trb1 to Trb5 are GaAs-HBTs; and Rbb1 to Rbb6 and Rb22 denote resistors.
The emitter follower circuit including Trb1 supplies voltage corresponding to reference voltage to the input terminal of the amplifier element Tr2. The RF signal inputted from the terminal RFin are inputted to the base of the amplifier element Tr via the capacitor C4 of an input matching circuit. The amplified RF signals are outputted from the collector of the amplifier element Tr2 to the terminal RFout. The bias circuit operates so as to keep the idle current of the amplifier element constant against change in temperature (for example, refer to Japanese Patent Application Laid-Open No. 2004-343244).